1. Field of the Invention
The present invention relates generally to the field of inkjet printing and, more particularly, to the delivery of ink to inkjet print heads and, more specifically to a regulator for a free-ink inkjet pen.
2. Description of Related Art
Inkjet technology is relatively well developed. The basics of this technology are described by W. J. Lloyd and H. T. Taub in "Inkjet Devices," Chapter 13 on Output Hardcopy Devices (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988) and in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4, (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994).
The typical thermal inkjet print head has an array of precisely formed nozzles attached to a print head substrate that incorporates an array of firing chambers that received liquid ink (i.e., colorants dissolved or dispersed in a solvent) from an ink reservoir. In what is sometimes referred to as a disposable print cartridge, the ink reservoir is an integral element with the print head, sometimes referred to as on-axis. Alternatively, the pen can be a free-ink type print mechanism, where the ink is supplied to the print head mechanism from a separate, self-contained ink supply such as a biased ink bladder or bag--as shown in U.S. Pat. No. 5,359,353 (Hunt et al.) assigned to the common assignee of the present invention and incorporated herein by reference--sometimes referred to in the art as off-axis. An on-axis regulator mechanism is provided with the pen to control ink flow and print head pressure; one such regulator mechanism is disclosed by S. Dana Seccombe et al. in U.S. patent application Ser. No. 08/409,255, a continuation-in-part of Ser. No. 08/065,957 for an Apparatus for Providing Ink to a Printhead (sic).
Each firing chamber has a thin-film resistor, known as a firing resistor or heater resistor, located opposite the nozzle such that ink can collect between the heater resistor and the nozzle. When electric printing pulses heat the print head firing resistor, a small portion of the ink near it vaporizes and ejects a drop of ink from the print head via a nozzle orifice. The nozzles are arranged in a matrix array. Properly sequencing the operation of each firing resistor causes alphanumeric characters or graphics images to form on paper as the print head is scanned across adjacently positioned print media and a dot matrix of ink drops is printed to form a graphics image and alphanumeric characters.
In an effort to reduce the cost and size of inkjet printers and to reduce the cost per printed page, engineers have developed inkjet printers having small, moving print heads that are connected to large stationary ink reservoirs by flexible ink tubes. This development is called "off-axis" printing. In such printers the mass of the print head is sharply reduced so that the cost of the print head drive system and the overall size of the printer can be minimized. In addition, separating the ink reservoir from the print head has allowed the ink to be replaced as it is consumed without requiring frequent replacement of the costly print heads.
With the development of off-axis printing has come the need for numerous flow restrictions to the ink between the ink reservoir and the print head. These restrictions include additional orifices, or ink ports, narrow conduits, and shut-off valves. To overcome these flow restrictions and also to provide ink drops suitably over a range of printing speeds, ink is now transported to the print head at an elevated pressure and a pressure reducer has been added to deliver the ink to the print head at an optimum back pressure (an internal pressure gauged at the print head that is substantially less than the pressure at the ink reservoir and through the conduits).
One complication in the evolution of off-axis printing is the increasing need to maintain the back pressure of the ink at the print head to within as small a range as possible. Changes in back pressure greatly affect print density and print quality, and major changes in back pressure can cause either the ink to drool out of the nozzles or to deprime the print cartridge.
There are several causes for such changes in back pressure. One cause occurs when air is entrapped within the print cartridge and the print cartridge is subjected to changes in environmental parameters such as altitude, acceleration, and temperature ("air" as used herein includes both ambient air and gases that are out gassed from the ink chemical itself. If the air entrapped in a print pen acts according to the Ideal Gas Law, PV=nRT, then any changes in any of these parameters will cause corresponding changes in print head back pressure. Another cause is the delay between the time the print head starts to eject ink during on-demand printing and the time the pressure regulator actuates to restore the back pressure.
These complications as well as the use of pressurized ink delivery have all resulted in a need for more accurate back pressure regulation at inkjet print heads and for more precise compensation techniques.